Attention has been directed to clean hydrogen energy in recent years owing to the depletion of fossil fuels such as petroleum and global environmental problems due to increases in carbon dioxide. In view of the situation, hydrogen absorbing alloys which are reversibly reactive with a large quantity of hydrogen with heat of reaction are regarded as indispensable hydrogen storage media and energy conversion media.
Ever since LaNi5 alloy of CaCu5-type crystal structure was disclosed (JP-B-34315/1974), various hydrogen absorbing alloys of the CaCu5-type having the same structure as the disclosed alloy have heretofore been proposed (see, for example, JP-B-28626/1984).
However, this LaNi5 hydrogen absorbing alloy has the problem of being low in hydrogen gas absorbing-desorbing reaction rate. Accordingly, the alloy is usable free of trouble as a hydrogen storage medium for fuel sources, such as fuel cells, wherein the frequency of hydrogen absorption or desorption is generally once every hour, but fails to fully exhibit its performance because of the low reaction rate when used with a hydrogen absorption or desorption frequency of several times per hour as is the case with heat pumps.
To overcome this drawback, an MmNi5 alloy (Mm: misch metal) is provided in which Ni is partly replaced by Mn to thereby enhance the activity of the alloy on hydrogen and improve the reaction rate (see, for example, JP-B-10543/1986).
This alloy nevertheless is not only limited in composition but also has the drawback of failing to exhibit other important characteristics required of hydrogen absorbing alloys, e.g., the reversibility of reaction and hydrogen absorption-desorption cycle life characteristics.
An object of the present invention is to provide a hydrogen absorbing alloy which is increased in reaction rate without being restricted in composition and which is unimpaired in the reversibility of reaction and hydrogen absorption-desorption cycle life characteristics.